There are various reduction processes for producing reduced iron or alloy iron. Among these, there is the process using powder metal oxides as feed materials to produce pellets and reducing these pellets at a high temperature.
This type of process is performed using a reduction furnace. As examples of such a reduction furnace, there are a shaft type hydrogen gas reduction furnace, rotary kiln type reduction furnace, rotary hearth type reduction furnace, etc.
Among these, a shaft type hydrogen gas reduction furnace reduces pellets made from powder ore by hydrogen gas.
On the other hand, in a rotary kiln type reduction furnace and rotary hearth type reduction furnace, heat is supplied to the reduction furnace and a reduction reaction is caused by the carbon mixed into the pellets. That is, in a rotary kiln type reduction furnace and rotary hearth type reduction furnace, shaped articles (pellets) obtained by mixing and shaping coal, coke, or other carbon and powder metal oxides are reduced.
The method of producing reduced iron using a rotary kiln type reduction furnace or rotary hearth type reduction furnace can use inexpensive coal etc., so is drawing attention as an economical method of producing reduced iron.
A rotary hearth type reduction furnace is a furnace of a type where a disk-shaped refractory hearth with a center cut away is rotated on rails at a constant speed under a fixed refractory ceiling and side walls. The diameter of the rotating hearth is 10 to 15 meters, while the width of the hearth is 2 to 6 meters.
While the hearth is rotating, the feed material feeder, heat zone, reduction zone, and product ejector are successively moved. The shaped articles of the feed material are charged from the feed material feeder. After this, the shaped articles are heated in the heat zone to about 1200° C. or more, then the carbon and metal oxides react in the reduction zone whereby a metal is produced.
In a method of reduction using a rotary hearth, the heating is quickly performed by radiation, so the reduction reaction ends in 7 to 20 minutes. The reduced shaped articles are ejected from the furnace and cooled, then used as a feed material for an electric furnace or blast furnace.
In this way, in a rotary hearth type reduction furnace, powder mainly comprised of carbon and metal oxides is used as the shaped articles for reduction by heating. In general, powders of at least three types of feed materials are used. This is to adjust the ratio between the metal oxides and carbon and adjust the composition of the particles sizes when producing the shaped articles.
In operation, these feed materials are mixed to produce the shaped articles. At this time, the feed material powders are mixed in a predetermined ratio so as to ensure a suitable chemical composition and particle size composition. The result is shaped by a molding machine.
In the method of reduction using a rotary hearth type reduction furnace, ore is generally used as the powder containing the metal oxides, but sometimes the dust or sludge produced in the metal refining process or processing process is used.
In particular, the dust or sludge produced in the ferrous metal industry includes zinc, lead, and other impurities. These evaporate at a reduction temperature of 1200° C. or more. Therefore, a rotary hearth type reduction furnace is an effective means for removing impurities.
In this way, in a rotary hearth type reduction furnace, the zinc, lead, and other impurities mixed in the shaped articles (pellets) form dust components in the exhaust gas. When the concentration of zinc or lead in the dust is high, the dust is used as a zinc feed material or lead feed material in nonferrous metal refineries.
To enable stable operation of a rotary hearth type reduction furnace, it is important to suitably adjust the chemical composition of the shaped articles (pellets). In regard to the operation, at the time of reducing the most general iron oxide, the amounts of the iron oxide and carbon, mainly the ratio of the iron oxide and carbon, are important as ingredients to be adjusted.
When using a powder feed material including, in addition to zinc and lead, alkali metals and halogen elements mixed in, special consideration is required for volatile substances mixed into the iron oxide in addition to adjustment of the ratio of the iron oxide and carbon.
The inventors, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2003-090686 (Japanese Patent Application No. 2001-279055 of the same inventors), found that if the exhaust gas of a rotary hearth type reduction furnace contains a large amount of sodium chloride, potassium chloride, or other alkali metal halides, due to the (i) problem that these substances deposit as dust inside the exhaust gas treatment system and become factors inhibiting operation and the (ii) problem that the concentration of zinc in the dust falls and the value as a zinc feed material falls, it is important to prepare the feed material under the following conditions.
That is, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2003-090686, the total number of moles (A) of zinc and lead in the feed material, the total number of moles (B) of potassium and sodium, and the total number of moles (C) of chlorine and fluorine are made to satisfy the relationship of (0.8C-0.7B)/A<0.36. By preparing the feed material to satisfy these conditions, the deposition of dust inside the exhaust gas treatment system can be suppressed and stable operation over a long period of time becomes possible.
Further, as shown in Japanese Unexamined Patent Publication (Kokai) No. 2000-169906, the inventors have proposed to improve the structure of the exhaust gas treatment system and control the exhaust gas temperature so as to suppress deposition of dust. By combining these measures against dust deposition and the restrictions in feed material ingredients in the invention described in Japanese Unexamined Patent Publication (Kokai) No. 2003-090686, it is possible to maintain a stable operating state of the exhaust gas treatment system.
Accordingly, in the prior art, the feed material for use in a rotary hearth type reduction furnace was analyzed for composition in advance and the conditions on the feed material such as the contents of zinc and lead and halogen elements and alkali metals were restricted and the exhaust gas treatment system modified so as to solve the above problems.
However, the method of ensuring stable operation while preventing dust deposition in an exhaust gas treatment system of a rotary hearth type reduction furnace by restricting the contents of the zinc and lead and the halogen elements and alkali metals in the feed material suffers from the problem that the feed material is restricted to feed material containing relatively little amounts of halogen atoms and alkali metals.
For example, when treating dust containing iron oxide or pickling sludge of steel products produced at steelmaking plants, the content of the potassium chloride or sodium chloride in these feed materials is high and application of the method described in Japanese Unexamined Patent Publication (Kokai) No. 2003-090686 is difficult.
For example, the dust contained in blast furnace gas often contains a total of 1 mass % of potassium chloride or sodium chloride. Further, the sludge produced by pickling of steel products often contains the hydrochloric acid or fluoric acid used at the time of pickling and residual matter. In both cases, there is the problem that the above feed material conditions cannot be satisfied.
As a result, when treating dust or sludge by a rotary hearth type reduction furnace, the problem has arisen of the dust depositing at the gas passages of the boiler or heat exchanger for reclaiming waste heat and other locations inside the waste gas treatment system. That is, the zinc oxide or lead oxide end up containing a certain percentage or more of alkali metals and halogen elements.
Some of the dust generated from a rotary hearth type reduction furnace (hereinafter referred to as “secondary dust”) contains alkali metals and halogen elements in a high rate of 20 to 45 mass %. As a result of this high rate of content, an inorganic mixture containing zinc oxide, zinc chloride, sodium chloride, potassium chloride, etc. mixed together is formed. This substance has a low melting point of 600° C. or less.
The secondary dust containing alkali metals and halogen elements in high concentrations exhibits an extremely high deposition ability under conditions of 400 to 600° C. This deposits on the gas passages of the boiler or heat exchanger to clog the exhaust gas channels and obstruct operation of the rotary hearth type reduction furnace.
In this way, in the case of a feed material containing large amounts of alkali metals and halogen elements, with just the prior art, these elements had a detrimental effect and stable operation was not possible.
Note that this dust deposition ability becomes higher when the ratio of the sodium chloride, potassium chloride, etc. with respect to the zinc oxide (partially zinc chloride) is high. Further, it becomes higher when the ratio of the sodium chloride, potassium chloride, etc. themselves is high.
Further, in a rotary hearth type reduction furnace, when performing recycling treatment consisting of introducing a carbon-bearing material or other reductant to the dust, sludge, or other steelmaking waste comprised mostly of iron oxide, but also containing a large amount of zinc and heating the result to reduce, evaporate away, and reoxidize the zinc oxide (ZnO) contained in the steelmaking waste so as to thereby recover it at a dust collector as secondary dust, the secondary dust will contain zinc in a high concentration, so the secondary dust can be utilized as a zinc feed material.
If the concentration of zinc in the zinc-containing secondary dust (converted to metallic zinc, hereinafter referred to as T. Zn, the same for the concentration of lead, which is converted to metallic lead and referred to as T. Pb) is 50 to 55%, the quality becomes one able to be directly utilized for a zinc blast furnace. This secondary dust is valuable as a zinc feed material.
However, when using such a feed material containing large amounts of alkali metals and halogen elements, the sodium chloride, potassium chloride, etc. of the feed material migrate to the secondary dust and cause the problem of a drop in the zinc concentration of the secondary dust.
In some cases, the concentration of alkali metals and halogen elements becomes 30 mass % or more and the T. Zn becomes a low 30 to 40 mass %, whereupon the secondary dust can no longer be directly used in a zinc blast furnace.
In particular, halogen elements inhibit the reaction at the time of zinc refining, so restriction of their quantity is an important item in management in zinc refining.
That is, to recycle recovered secondary dust as a zinc feed material, it is necessary to remove harmful substances from the low zinc concentration, high halogen concentration secondary dust by pre-treatment to concentrate the zinc. Further, this pre-treatment requires massive costs, so the cut-cutting and energy-saving effects to be inherently enjoyed due to recovery of zinc oxide are reduced and, in the worst case, cancelled out entirely.
Therefore, conventionally, only steelmaking waste with little chlorine content is selected as a feed material. Steelmaking waste with a high chlorine content has been deemed to have no merits for treatment and therefore has not been used as a recycling material.
Further, substantially the same problems occurred as with zinc when recycling lead as well.
As technology for recovering zinc, lead, and other valuable metals from dust including iron oxide, for example, Japanese Examined Patent Publication (Kokoku) No. 53-29122 discloses technology comprised of a step of washing the dust etc. to remove the chlorine, sodium, and potassium, a step of adding coke to the washed dust obtained at this step and granulating and sintering the same to obtain sintered iron ore containing zinc and lead, and a step of washing the sintered dust obtained by removing the dust from the sintering gas from this step by alkali water to remove the fluorine to obtain nonferrous metal slag containing lead and cadmium.
However, this technology, as described in Japanese Examined Patent Publication (Kokoku) No. 53-29122 (see page 3, column 6), does not reduce or vaporize the zinc in the sintering step, but leaves it in the sintered ore, so it is necessary to separately prepare a standing type distillation furnace etc. to reduce, vaporize, and recover the zinc.
Further, in this technology, the dust washed by the water contains 30 to 40% moisture, so as described in this publication (see page 2, column 4), it is necessary to dry it in a rotary drier etc. before sintering.
PCT Publication Pamphlet WO 01/42516 A1 discloses mixing by agitation a powder containing metal oxides and carbon in a state containing at least 1.0 times the moisture with respect to the powder weight, dehydrating this by a dehydration system until a moisture content of 16 to 26 mass %, then shaping it by a compression molding machine to produce shaped articles of a powder filling rate of 0.43 to 0.58, charging the shaped articles into a rotary hearth type reduction furnace having an atmospheric temperature of 1170° C. or less, and reducing them by sintering at 1200° C. or more.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2001-303115 discloses technology of dehydrating a slurry of a mixture of powder containing metal oxides and powder containing carbon by a double-roll press type dehydrator down to a moisture content of 16 to 27%, producing shaped articles by an extrusion molding machine, and reducing by sintering the articles by a rotary hearth type reduction furnace to obtain metal.
However, in this technology, the high moisture content powder is charged into the reduction furnace without any drying step, so removal of the sodium chloride, potassium chloride, and other volatile harmful substances is not considered.
In this way, when using a rotary hearth type reduction furnace to treat a feed material containing large amounts of alkali metals and halogen elements, the following problems occur. In particular, there were problems in stable operation of the rotary hearth type reduction furnace and the pre-treatment for converting the secondary dust to a good quality zinc feed material.
Therefore, new technology enabling stable operation and pre-treatment of secondary dust economically even when using a feed material containing large amounts of alkali metals and halogen elements has been sought.